1. Field of the Invention
The present invention relates to a device for testing a surface, comprising a holder; a transmitter supported by the holder, the transmitter being able to transmit an electromagnetic signal to the surface at a transmission frequency; a receiver for receiving a signal reflected on the surface. Such a device is intended to test the heterogeneity and/or local defects that may be present on surfaces.
In particular, such a device is designed to determine the radioelectric homogeneity of the surface, by determining the variations in the radioelectric properties of the surface using a non-destructive test method.
2. Description of the Related Art
Good radioelectric surface homogeneity may be required in many applications, in particular those designed to produce and incorporate antennas, in the insulation of equipment sensitive to strong fields, and in reducing the diffusion of waves lighting those surfaces.
In particular, the parabolic reflectors used in certain high-gain antennas must have a homogenous surface conductivity. Without such homogeneity, the conductivity defects cause dispersions of the electromagnetic radiation that increase the standing wave levels and amplify the interactions between adjacent systems.
The radomes present across from the antennas must on the contrary be radioelectrically transparent. Transparency discontinuities decrease the intensity of the signals transmitted and received and diffuse the waves outside the angular sectors of interest.
Furthermore, in the case where part of the antenna system must be isolated, an absorbent material may be used to cover the antenna system. It is then necessary for the surface reflectivity to be homogenous.
To detect radioelectric surface defects, it is known to examine the variations of the radioelectric properties of the surfaces.
In that context, non-destructive techniques are used. Among these techniques, it is known to use Foucault currents using a coil by creating a magnetic field near the surface penetrating the material. This penetration creates induced currents that are picked up by the coil or measured by an adjacent coil.
The induced currents result from the conductivity and magnetic permeability of the material and influence the impedance specific to the coil or the mutual impedance between two coils of the device.
In certain cases, to improve the measuring quality, it is known to perform localized electromagnetic reflectivity measurements. These measurements may be done using an antenna and a reflector, or using a waveguide.
Radar techniques may also be used by moving a pair of transmission and receiving antennas relative to a scene to obtain a spatial resolution along the movement axis.
However, the existing techniques have many limitations. Thus, the technique using Foucault currents only makes it possible to test conductive or magnetic surfaces. Furthermore, for frequencies above 300 MHz, the coils used as sensors generate spatial radiation and become sensitive to echoes outside the area of interest.
Local electromagnetic reflectivity measurements are limited in terms of resolution, in particular in light of the width of the lighting area, which is typically in the vicinity of a half-wavelength. These measuring methods are therefore not suited to detecting localized impedance discontinuities, for example resulting from a slit related to a crack in a metallization layer.
Radar techniques are expensive and sometimes difficult to carry out, in particular to avoid outside echoes that saturate the measurement. It is generally necessary to perform the measurements in an anechoic enclosure.
Furthermore, to obtain a good spatial resolution on the detection of the defects, the measuring frequency band must be wide, which leads to significant measuring times.